This invention relates to a hollow fiber membrane contactor for phase separations and other phase contact applications. The contactor is made from perfluorinated thermoplastic polymeric materials, has a high packing density providing high useful contacting area, and the ability to operate with liquids of low surface tension.
Liquid-gas contactors are used to transfer one or more-soluble substances from one phase to another. Examples of conventional contactors are packed towers, plate columns and wetted wall columns. In these systems, gas absorption of one or more components from the gas stream is accomplished by dispersing the gas as bubbles in packed towers and plate columns in a countercurrent flow to the liquid stream. Absorption efficiency is controlled apart from solubility considerations by the relative rate of the flows and the effective surface area of the gas flow bubbles. In wetted wall contactors the gas stream flows past a downward flow of liquid on the inside wall of a vertical tube. Gas stripping is used to transfer a gas dissolved in liquid into a gas stream. Similar contactors are used for gas stripping.
Conventional contactors have several deficiencies. Primary among these is the fact that the individual gas and liquid flows cannot be varied independently over wide ranges. Tray columns are prone to such problems as weeping at low gas flows and flooding at high liquid flows. Packed towers can flood at high flow rates. The use of low liquid flow rates In a packed tower would lead to channeling and reduced effective surface area. Excessive frothing or foam formation can lead to process inefficiency. Wetted wall contactors have inherently low mass transfer coefficients, and can flood at high gas flow rates. The development of membrane contactors has overcome these deficiencies.
Membrane contactors are devices through which two fluid phases flow separated by a membrane permeable to the gas being transferred. If a microporous membrane is being used, the preferred method relies on the non-wetting characteristic of the membrane material and the pore size to prevent liquid from intruding into the pores and filling them. Gas transfer then occurs through the gas filled pores to or from the liquid, depending on whether the process is absorption or stripping. If a non-porous membrane is used, gas transfer is controlled by the diffusion rate in the non-porous layer of the membrane. In some cases, such as oxygen removal from ultrapure water, gas stripping is done with a vacuum instead of a stripping gas flow. While other membrane geometries are available for this use, hollow fiber membranes are suited as contactors.
A hollow fiber porous membrane is a tubular filament comprising an outer diameter, an inner diameter, with a porous wall thickness between them. The inner diameter defines the hollow portion of the fiber and is used to carry one of the fluids. For what is termed tube side contacting, the liquid phase flows through the hollow portion, sometimes called the lumen, and is maintained separate from the gas phase, which surrounds the fiber. In shell side contacting, the liquid phase surrounds the outer diameter and surface of the fibers and the gas phase flows through the lumen.
The outer or inner surface of a hollow fiber membrane can be skinned or unskinned. A skin is a thin dense surface layer integral with the substructure of the membrane. In skinned membranes, the major portion of resistance to flow through the membrane resides in the thin skin. The surface skin may contain pores leading to the continuous porous structure of the substructure, or may be a non-porous integral film-like surface. In porous skinned membranes, permeation occurs primarily by connective flow through the pores. Asymmetric refers to the uniformity of the pore size across the thickness of the membrane; for hollow fibers, this is the porous wall of the fiber. Asymmetric membranes have a structure in which the pore size is a function of location through the cross-section, typically, gradually increasing in size in traversing from one surface to the opposing surface. Another manner of defining asymmetry is the ratio of pore sizes on one surface to those on the opposite surface.
Manufacturers produce membranes from a variety of materials, the most general class being synthetic polymers. An important class of synthetic polymers are thermoplastic polymers, which can be flowed and molded when heated and recover their original solid properties when cooled. As the conditions of the application to which the membrane is being used become more severe, the materials that can be used become limited. For example, the organic solvent-based solutions used for wafer coating in the microelectronics industry will dissolve or swell and weaken most common polymeric membranes. The high temperature stripping baths in the same industry consist of highly acid and oxidative compounds, which will destroy membranes made of common polymers. Perfluorinated thermoplastic polymers such as poly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) (poly(PTFE-CO-PFVAE)) or poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) are not adversely affected by severe conditions of use, so that membranes made from these polymers would have a decided advantage over ultrafiltration membranes made from less chemically and thermally stable polymers. These thermoplastic polymers have advantages over (poly(tetrafluoroethylene) (PTFE), which is not a thermoplastic, in that they can be molded or shaped in standard type processes, such as extrusion. Perfluorinated thermoplastic hollow fiber membranes can be produced at smaller diameters than possible with PTFE. Fibers with smaller diameters, for example, in the range of about 350 micron outer diameter to about 1450 micron outer diameter, can be fabricated into contactors having high membrane surface area to contactor volume ratios. This attribute is useful for producing compact equipment, which are useful in applications where space is at a premium, such as in semiconductor manufacturing plants.
Being chemically inert, the Poly (PTFE-CO-PFVAE) and FEP polymers are difficult to form into membranes using typical solution casting methods as they are difficult to dissolve in the normal solvents. They can be made into membranes using the Thermally Induced Phase Separation (TIPS) process. In one example of the TIPS process a polymer and organic liquid are mixed and heated in an extruder to a temperature at which the polymer dissolves. A membrane is shaped by extrusion through an extrusion die, and the extruded membrane is cooled to form a gel. During cooling the polymer solution temperature is reduced to below the upper critical solution temperature. This is the temperature at or below which two phases form from the homogeneous heated solution, one phase primarily polymer, the other primarily solvent. If done properly, the solvent rich phase forms a continuous interconnecting porosity. The solvent rich phase is then extracted and the membrane dried.
Hydrophobic microporous membranes are commonly used for contactor applications with an aqueous solution that does not wet the membrane. The solution flows on one side of the membrane and a gas mixture preferably at a lower pressure than the solution flows on the other. Pressures on each side of the membrane are maintained so that the liquid pressure does not overcome the critical pressure of the membrane, and so that the gas does not bubble into the liquid. Critical pressure, the pressure at which the solution will intrude into the pores, depends directly on the material used to make the membrane, inversely on the pore size of the membrane, and directly on the surface tension of the liquid in contact with the gas phase. Hollow fiber membranes are primarily used because of the ability to obtain a very high packing density with such devices. Packing density relates to the amount of useful membrane surface per volume of the device. It is related to the number of fibers that can be potted in a finished contactor. Also, contactors may be operated with the feed contacting the inside or the outside surface, depending on which is more advantageous in the particular application. Typical applications for contacting membrane systems are to remove dissolved gases from liquids, xe2x80x9cdegassingxe2x80x9d; or to add a gaseous substance to a liquid. For example, ozone is added to very pure water to wash semiconductor wafers.
Porous contactor membranes are preferred for many applications because they will have higher mass transfer than nonporous membranes. For applications with liquids having low surface tensions, smaller pore sizes will be able to operate at higher pressures due to their resistance to intrusion. Non-porous contactor membranes are preferred in cases where the liquid vapor pressure is high, or where high temperature operation increases the vapor pressure. In these cases, evaporation through a porous membrane may result in substantial liquid loss. Non-porous membranes may also be preferred in high-pressure applications, where intrusion of a porous membrane would be a problem.
Membrane contactors can also be useful in applications where in addition to phase transfer of a species from a feed stream to a second phase, a chemical reaction is desired between that species and a second reactant in the second phase. Membrane contactors would provide high surface area for mass transfer and maintain the product separate from the feed stream.
Z. Qi and E. L. Cussler (J. Membrane Sci. 23(1985) 333-345) show that membrane resistance controls absorption of gases such as ammonia, SO2 and H2S in sodium hydroxide solutions. This seems generally true for contactors used with strong acids and bases as the absorption liquid. For these applications, a more porous contactor membrane, such as a microporous membrane, would have an advantage, because the membrane resistance would be reduced. This would be practical if the liquid does not intrude the pores and increase resistance. With the very low surface tension materials used in the present invention, this would be possible without coating the surface of the fibers with a low surface tension material, which is an added and complex manufacturing process step.
Membrane contactors have several advantages over conventional equipment. Membrane contactors have a higher surface area per unit volume than conventional packed towers. More importantly, membrane contactors are not disturbed by high or low flow rates and do not suffer from the problems set forth above for conventional contactors. This is due to the fact that in membrane contactors, the flow rates can be controlled independently because the separate phases are not in physical contact and cannot influence each others flow. Membrane contactors also have the advantage that bubbles are not formed in the liquid stream under proper operating conditions. These advantages are useful in important applications.
Ozone treatment of drinking water is being increasingly considered. Ozone has the capability of eliminating all viruses, and does not form substances such as trihalomethanes, which are by-products of chlorine treatment and natural substances such as humic or fulvic acids which may be present. For applications requiring a compact apparatus, such as at remote sites, the higher efficiency of a membrane contactor would be preferable to the typical small bubble diffuser, which requires significant water-ozone depth to be effective.
In the manufacturing process for integrated circuits, a photoresist coating is baked onto a silicon wafer that must be removed after processing. Oxidation is a commonly used method to clean the wafers.
U.S. Pat. No. 5,082,518 describes a sulfuric acid and oxidizer process to clean semiconductor wafers. A gas distribution system comprising a sparger plate with diffusion holes distributes ozone directly into a treatment tank containing sulfuric acid. This system has the disadvantages of lower absorption efficiency of ozone absorption into the water due to the large bubbles of ozone produced. The contactor of the present invention has the chemical stability to operate directly in harsh environments and would improve cleaning reaction efficiency by supplying a bubble free ozone solution.
Ohmi et al, J. Electrochem. Soc., Vol. 140, No. 3, March 1993, pp. 804-810, describe cleaning organic impurities from silicon wafers at room temperature with ozone-injected ultrapure water. U.S. Pat. No. 5,464,480 shows that ozone diffused through a subambient temperature deionized water will quickly and effectively remove organic materials such as photoresist from wafers without the use of other chemicals. It is believed that lowering the temperature of the solution enables a sufficiently high ozone concentration in solution to substantially oxidize all of the organic material on the wafer to insoluble gases. The means for diffusing a gas can be any means which provides fine bubbles of ozone or other gases into the tank and uniformly distributes the gas throughout the tank.
In U.S. Pat. No. 5,464,480, preferably, the bubbles that are provided by the diffuser are initially about 25 to about 40 microns in diameter. The gas diffuser preferably is made from a mixture of polytetrafluoroethylene (PTFE) and perfluoroalkoxylvinylether. By varying the temperature and pressure under which the mixture is prepared by methods known in the art, both porous and nonporous members are formed. The impermeable and permeable members are preferably comprised of about 95% PTFE and about 5% perfluoroalkoxylvinylether. The permeable member and the impermeable member may be joined by any number of methods as long as the result is a composite member that will not come apart under the stresses in the tank. Preferably, the members are heat sealed together, essentially melting or fusing the members together using carbon-carbon bonds. Once the permeable member is formed, a trench is bored out of the PTFE in the top portion of the member. The resulting diffuser has on the order of about 100,000 pores of a size of about 25 to about 40 microns in diameter through which gas may permeate into the treatment tank. The use of the trench in the diffuser allows the gas to diffuse into the tank as very fine bubbles. In applications for the semiconductor manufacturing industry, a device that supplied homogeneous bubble free ozone dissolved in ultrapure water would provide more efficient oxidation reactions because the reaction would not be localized at the bubbles. The more homogeneous solution would provide for a more uniform cleaning reaction. Furthermore, the high surface area to volume ratio inherent in hollow fiber devices would give a compact system, suitable for semiconductor operations.
Dissolved oxygen in ultrapure water is another problem in semiconductor device manufacturing. Oxygen removal to less than one part per billion (ppb) is required to prevent uncontrolled oxide growth. Potential problems associated with uncontrolled oxide growth are prevention of low temperature epitaxy growth, reduction of precise control of gate-oxide films, and increased contact resistance for VIA holes. This uncontrolled growth can be overcome by stripping dissolved oxygen to less than 1 ppb from the ultrapure water used in the manufacturing process. The high packing density and cleanliness associated with an all perfluorinated thermoplastic contactor are advantages in such applications.
U.S. Pat. No. 5,670,094 provides an oxidized water producing method in which a pressurized ozone gas generated by an electric discharge type ozonator is dissolved in water to be treated through a hollow fiber membrane, characterized in that the water pressure inside the membrane is maintained higher than the pressure of the ozone gas supplied to the outside of the hollow fiber membrane to prevent tiny bubbles and impurities from getting mixed into the water being treated, and the ozone concentration in the treated water is controlled on the basis of the concentration of the ozone gas. This method reference discloses only PTFE membranes and does not contemplate the use of an all perfluorinated thermoplastic contactor.
Commercially available all PTFE hollow tube contactors are referred to as xe2x80x9chollow tubesxe2x80x9d, probably because they are relatively large. Patent JP7213880A discloses the fiber manufacturing process for making composite PTFE hollow tubes for ozonizing applications. The first step of this process involves extruding PTFE paste derived from a mixture of PTFE powder and lubricants. After the tube is formed, the lubricants are extracted and the powder sintered into a slightly porous PTFE solid tube. The tube is then stretched longitudinally to make it porous. This is different than typical PTFE flat sheet membranes made by a similar process. To generate really fine microporous structures, characterized by a node to fibrils network, most PTFE membranes are made by biaxial stretching. For hollow fibers, the equivalent process would have been stretching the fiber radially. Probably because of the impracticality of such a step, this radial stretching step is missing from the disclosed process. Consequently, the pores in this tube are only xe2x80x9chalf-formedxe2x80x9d, i.e. it did not attain the xe2x80x9cnode to fibril networkxe2x80x9d of flat sheet membrane. To compensate for this deficiency, the tube underwent a second step of laminating a regular microporous flat sheet membrane on top of the external surface of the porous tube. This step involves lamination of a long narrow strip of PTFE microporous membrane spirally on the surface of the tubing. This is a tedious, labor intensive process. Also, with the membrane laminated to the outside of the hollow tube, the resistance to mass transfer in tube-side flow could be higher in cases where the fluid partially intrudes into the support layer. This arrangement diminishes the potential of using the membrane as the barrier for separating the two fluid phases. These deficiencies are overcome with the hollow fiber membranes of the present invention.
An advantage for contacting applications is that the very low surface tension of these perfluorinated polymers allows use with low surface tension liquids. For example, highly corrosive developers used in the semiconductor manufacturing industry may contain surface tension reducing additives, such as surfactants. These developers could not be degassed with typical microporous membranes because the liquid would intrude the pores at the pressures used and permeate, causing solution loss and excess evaporation. In addition, liquid filling the pores would greatly add to the mass transfer resistance of gas transport. U.S. Pat. No. 5,749,941 describes how conventional hollow fiber membranes of polypropylene or polyethylene cannot be used in carbon dioxide or hydrogen sulfide absorption into aqueous solutions containing an organic solvent without the use of a solution additive to prevent leakage. While polytetrafluoroethylene (PTFE) membranes would work in these applications, presumably because of their lower surface tension, they are difficult to process into hollow fibers. The membranes of the present invention are made from polymers having similar surface tension properties to PTFE and are more readily manufactured into small diameter hollow fiber membranes.
WO 9853894 describes a process of forming compact, high flux, fouling resistant gas filters by coating continuous ultra thin layer of non-porous gas permeable polymer over filter surface by contacting one side of microporous substrate with dilute coating solution of polymer, preferably amorphous copolymer of perfluoro-2,2-dimethyl-1,3-dioxole, which is hydrophobic and oleophobic. Substrate pore size filters polymer from solution as solvent flows through, leaving ultra thin layer of polymer. Process is useful to coat sheet and hollow fiber substrates, particularly multiple hollow fibers assembled in modules. These membranes have been described as useful for contactor applications. (S. Nemser, Paper presented at 1998 North American Membrane Society Meeting.) This method requires a separate and complex coating step to produce a non-porous contactor fiber. Furthermore, a fully perfluorinated thermoplastic contactor is not described.
In a first embodiment of the present invention, a highly asymmetric perfluorinated thermoplastic hollow fiber is used as the barrier. The membrane is comprised of a skinned surface on one diameter, and a porous surface on the other diameter. The smaller pored skinned surface of the asymmetric membrane is designed to face the liquid flow and offer the highest resistance to liquid intrusion. The thin skin offers low diffusional resistance, yet the small pores offer the highest intrusion resistance. Also, the perfluorinated surface has low interfacial energy, which further increases the resistance to liquid intrusion.
In a second embodiment, a perfluorinated thermoplastic microporous hollow fiber membrane is used as the barrier. These membranes are useful in applications where the membrane resistance to mass transfer may be controlling, or liquid intrusion into the pores is a lesser problem.
In a third embodiment, the skinned surface of the perfluorinated thermoplastic hollow fiber membrane is non-porous.
This invention provides for a fully perfluorinated thermoplastic hollow fiber membrane contactor with unitary end structures having a high packing density and capable of operating with liquids having interfacial surface tension of greater than about 20 mN/m at 20xc2x0 C. A manufacturing method for the contactor is provided and described.
The contactor is comprised of a bundle of substantially parallel hollow fiber membranes potted at both ends and having unitary end structure(s) with the housing containing the fibers. The perfluorinated thermoplastic hollow fiber membranes of this invention are made of polymers such as poly (tetrafluoroethylene-co-perfluoro (alkylvinylether)), poly (tetrafluoroethylene-co-hexafluoropropylene), or blends thereof. The contactor 2 has fluid inlets and outlet connections for the two fluids to be contacted. As illustrated in FIG. 1, fluid 1 enters the contactor 2 through the fiber lumens 3, traverses the interior of the contactor 2 while in the lumens 3, where it is separated from fluid 1 by the membrane, and exits the contactor 2 through the fiber lumens at connection 20. Fluid 4 enters the housing through connection 30 and substantially fills the space between the inner wall of the housing and the outer diameters of the fibers, and exits through connector 20.
In the first embodiment, the fibers are asymmetric skinned membranes having a porous skinned surface on one diameter and a porous surface on the other diameter, with the fiber wall between comprising a porous structure. The process of making a contactor of the first embodiment from asymmetric skinned perfluorinated thermoplastic polymers uses hollow fiber membranes made by the process described in concurrent U.S. patent application No. 60/117,854, filed Jan. 29, 1999, the disclosure of which is incorporated by reference. For use with liquid on the lumen of the fibers, the inner diameter will be made skinned, and for use with liquid on the outer or shell side of the fibers, the outer diameter of the fibers will be made skinned. The potted fibers are spaced closely, without any fiber crossover or fibers impinging on each other so tightly that they can not be forced apart by the liquid or gas flow. For liquid flow in the lumen, which is the preferred mode of operation for liquid-gas contacting, the fibers do not have to be spaced uniformly apart. This simplifies the contactor manufacturing process.
In the second embodiment, perfluorinated thermoplastic microporous hollow fiber membranes are made by a process disclosed in patent application No. 60/117,852 filed Jan. 29, 1999, the disclosure of which is incorporated by reference. Equivalent membranes made by another process could also be used. Preferred polymers are perfluorinated thermoplastic polymers, more specifically poly (tetrafluoroethylene-co-perfluoro (alkylvinylether)) (poly (PTFE-CO-PFVAE)), poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP), or blends of these polymers, which are dissolved in a solvent to give a solution having an upper critical solution temperature, and which when the solution is cooled, separates into two phases by liquid-liquid phase separation. Teflon(copyright) PFA is an example of a poly (tetrafluoroethylene-co-perfluoro (alkylvinylether)) in which the alkyl is primarily or completely the propyl group. FEP Teflon(copyright) is an example of poly (tetrafluoroethylene-co-hexafluoropropylene). Both are manufactured by DuPont, Wilmington, Del. Neoflon(trademark) PFA (Daikin Industries) is a polymer similar to DuPont""s PFA Teflon(copyright). A poly (tetrafluoroethylene-co-perfluoro (alkylvinylether))polymer in which the alkyl group is primarily methyl is described in U.S. Pat. No. 5,463,006. A preferred polymer is Hyflon(copyright) POLY (PTFE-CO-PFVAE) 620, obtainable from Ausimont USA, Inc., Thorofare, N.J.
In a third embodiment, the conditions of membrane making of the first embodiment are adjusted to produce a skinned asymmetric membrane with a non-porous skin. A preferred method is to increase the amount of polymer used in the solution used to make the membranes.
The fibers are made by a Thermally Induced Phase Separation (TIPS) method, in which polymer is dissolved in a solvent at high temperatures and extruded through an annular die into a cooling bath. The resulting gel fiber is wound as a continuous coil on a steel frame with the fibers substantially parallel and not touching. The frame and coil are placed in an extraction bath to remove the solvent from the gel fiber. After extraction, the fibers are annealed on the frame for about 24 hours and then cooled. The fibers are removed from the frame and the flat coil of fibers is re-annealed to relax the fibers and prevent shrinkage in the potting and bonding step. The fibers are removed from the annealing oven and cooled. They are then gathered into a cylindrical bundle and potted and bonded in a single step.
Potting is a process of forming a tube sheet having liquid tight seals around each fiber. The tube sheet or pot separates the interior of the final contactor from the environment. The pot is thermally bonded to the housing vessel in the present invention to produce a unitary end structure. The unitary end structure comprises the portion of the fiber bundle which is encompassed in a potted end, the pot and the end portion of the perfluorinated thermoplastic housing, the inner surface of which is congruent with the pot and bonded to it. By forming a unitary structure, a more robust contactor is produced, less likely to leak or otherwise fail at the interface of the pot and the housing. The potting and bonding process is an adaptation of the method described in U.S. patent application No. 60/ 117,853 filed Jan. 29, 1999, the disclosure of which is incorporated by reference.
Potting and bonding are done in a single step. An external heating block is used for potting one end at a time. The perfluorinated thermoplastic end seals are preferably made of poly (tetrafluoroethylene-co-perfluoro (alkylvinylether)) having a melting point of from about 250xc2x0 C. to about 260xc2x0 C. A preferred potting material is Hyflon(copyright) 940 AX resin, from Ausimont USA Inc. Thorofare, N.J. Low viscosity poly (tetrafluoroethylene-co-hexafluoropropylene) with low end-of-melt temperatures as described in U.S. Pat. No. 5,266,639 is also suitable. The process involves heating the potting material in a heating cup at around 275xc2x0 C. until the melt turns clear and are free of trapped bubbles. A recess is made in the molten pool of potting material that remains as a recess for a time sufficient to position and fix the fiber bundle and housing in place. Subsequently, the recess will fill with the molten thermoplastic in a gravity driven flow.
A unitary end structure, by which is meant that the fibers and the pot are bonded to the housing to form a single entity consisting solely of perfluorinated thermoplastic materials is prepared by first pretreating the surfaces of both ends of the housing before the potting and bonding step. This is accomplished by melt-bonding the potting material to the housing. The internal surfaces on both ends of the housing are heated close to its melting point or just at the melting point and immediately immersed into a cup containing powdered (poly (PTFE-CO-PFVAE)) potting resin. Since the surface temperature of the housing is higher than the melting point of the potting resins, the potting resin is then fused to the housing resinxe2x80x94a condition for bonding to occur. The housing is then taken out and polished with a heat gun to fuse any excess unmelted powder. Without this pretreatment step, the housing surfaces often detach from the potting surfaces because of absence of intermixing of the two resins.
The unitary end structure(s) is cut and the lumen of the fibers exposed. The potted surfaces are then polished further using a heat gun to melt away any smeared or rough potted surfaces. A solder gun can be used to locally remelt and repair any defective spot, sometimes with the help of a drop of melted resin.